Acetogenic microorganisms can produce ethanol from carbon monoxide (CO) through fermentation of gaseous substrates. Fermentations using anaerobic microorganisms from the genus Clostridium produce ethanol and other useful products. For example, U.S. Pat. No. 5,173,429 describes Clostridium ljungdahlii ATCC No. 49587, an anaerobic microorganism that produces ethanol and acetate from synthesis gas. U.S. Pat. No. 5,807,722 describes a process and apparatus for converting waste gases into organic acids and alcohols using Clostridium ljungdahlii ATCC No. 55380. U.S. Pat. No. 6,136,577 describes a process and apparatus for converting waste gases into ethanol using Clostridium ljungdahlii ATCC No. 55988 and 55989.
Processes for producing ethanol from carbon monoxide involve culturing the acetogenic bacteria on increasing amounts of CO over time. High or low levels of CO in the fermentation may result in lower productivity. As CO feed rates to the fermentor increase, dissolved CO concentrations in the fermentation medium may increase. The increase in dissolved CO concentration in the fermentation medium may result in CO-inhibition and decreased levels of productivity.